Morphodynamic behaviour of a mixed sand–gravel ebb-tidal delta: Deben estuary, Suffolk, UK

Marine Geology 225 (2006) 23 – 44 www.elsevier.com/locate/margeo

Morphodynamic behaviour of a mixed sand–gravel ebb-tidal delta: Deben estuary, Suffolk, UK Helene Burningham *, Jon French Coastal & Estuarine Research Unit, Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK Received 16 March 2005; received in revised form 31 August 2005; accepted 13 September 2005

Abstract The morphodynamics of inlets and ebb-tidal deltas reflect the interaction between wave and tidal current-driven sediment transport and significantly influence the behaviour of adjacent shorelines. Studies of inlet morphodynamics have tended to focus on sand-dominated coastlines and reference to gravel-dominated or dgravel-richT inlets is rare. This work characterises and conceptualises the morphodynamics of a meso-tidal sand–gravel inlet at the mouth of the Deben estuary, southeast England. Behaviour of the inlet and ebb-tidal delta over the last 200 yr is analysed with respect to planform configuration and bathymetry. The estuary inlet is historically dynamic, with ebb-tidal shoals exhibiting broadly cyclic behaviour on a 10 to 30 yr timescale. Quantification of inlet parameters for the most recent cycle (1981–2003) indicate an average ebb delta volume of 1  106 m3 and inlet cross-sectional area of 775 m2. Bypassing volumes provide a direct indicator of annual longshore sediment transport rate over this most recent cycle of 30–40  103 m3 yr 1. Short-term increases in total ebb-tidal delta volume are linked to annual variability in the north to northeasterly wind climate. The sediment bypassing mechanism operating in the Deben inlet is comparable to the debb delta breachingT model of FitzGerald [FitzGerald, D.M., 1988. Shoreline erosional–depositional processes associated with tidal inlets, in: Aubrey, D.G., Weishar, L. (Ed.), Hydrodynamics and Sediment Dynamics of Tidal Inlets. Springer-Verlag Inc., New York, pp. 186–225.], although the scales and rates of change exhibited are notably different to sand-dominated systems. A systematic review of empirical models of sand-dominated inlet and ebb-tidal delta morphodynamics (e.g. those of [O’Brien, M.P., 1931. Estuary tidal prisms related to entrance areas. Civil Engineering, 1, 738–739.; Walton, T.L., and Adams, W.D., 1976. Capacity of inlet outer bars to store sand. Proceedings of 15th Coastal Engineering Conference, 1919–1937.; Gaudiano, D.J., Kana, T.W., 2001. Shoal bypassing in mixed energy inlets: geomorphic variables and empirical predictions for nine South Carolina inlets. J. Coast. Res., 17, (2), 280–291.]) shows the Deben system to be significantly smaller yet characterised by a longer bypassing cycle than would be expected for its tidal prism. This is attributed to its coarse-grained sedimentology and the lower efficiency of sediment transporting processes. D 2005 Elsevier B.V. All rights reserved. Keywords: ebb-tidal delta; inlet; mixed energy; mixed sand–gravel; estuary; shoal; cyclical behaviour; timescale

1. Introduction

* Corresponding author. Tel.: +44 20 7679 2000. E-mail addresses: [email protected] (H. Burningham), [email protected] (J. French). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.09.009

Hydrodynamically, tidal inlets are primarily controlled by the interaction of waves, tides and rivers (Boothroyd, 1985), whilst the morphology and behaviour of associated sediment shoals are dependent also

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on the nature and magnitude of sediment supply (FitzGerald et al., 2002). Ebb-tidal deltas are important contributors to littoral sediment budgets (Hicks and Hume, 1996) and exert a significant influence on the morphodynamics of adjacent shorelines (FitzGerald, 1988; Hicks et al., 1999). Although inlets interrupt the continuity of longshore sediment transport, they enable sediment transfer from updrift to downdrift shorelines, in addition to sediment exchange between seaward (ebb) and landward (flood) shoals. The evolving understanding of inlet and ebb/flood delta morphology and function has related inlet dynamics to associated shoreline behaviour. Cycles of erosion and deposition on neighbouring shorelines are often linked to phases of growth and decay of ebb-tidal shoals and to switches in ebb- and flood-channel position (e.g. Oertel, 1977; Hume and Herdendorf, 1992; FitzGerald et al., 2002). Short-term inlet dynamics have been linked to spatial patterns of beach erosion and accretion extending up to 2 km alongshore (Hicks et al., 1999). Immediately adjacent shorelines are also influenced by the modifications to nearshore wave climate caused by ebb-tidal shoals and channels (Hubbard, 1975). Most studies of inlet morphodynamics have focused on sand-dominated coastlines. Gravel-dominated or dgravel-richT inlets are relatively rare, and it is unclear if their morphology and behaviour is similar to that of sand-dominated systems. Coarse beaches differ from sandy beaches in terms of structure, sediment transport and behaviour (Carter and Orford, 1993), and distinctive suites of beach forms and behaviour can be identified for mixed-sediment beaches comprising specific combinations of sand and gravel (Jennings and Shulmeister, 2002). Such distinctions are potentially important within mixed-sediment systems, where multiple wave- and tide-driven sediment transport pathways interact over a range of temporal scales. The sediment partitioning observed in beach environments, where gravel is preferentially sustained on the upper foreshore and sand occupies the lower foreshore and nearshore zone, may be replicated in mixed-sediment inlet systems, depending on sediment supply and size distribution. This might have implications for the spatial and temporal scale of sediment transfer through ebb-tidal shoals, and the temporal intermittency resulting from the contrasting sediment transport potential of wave and tidal processes. Estuarine inlets are widespread in the UK. The majority are sandy and inlets with significant gravel components are not common. Most coastal gravel fea-

tures in the UK are associated with barriers fronting or enclosing brackish or freshwater wetlands and lagoons (e.g. Slapton and Porlock in Devon; Cley in North Norfolk; Dungeness in Kent; and Culbin, Moray). Gravel-rich barriers tend to be wave-dominated, and tidal exchange with backbarrier environments is often incapable of naturally maintaining a permanent inlet (e.g. Pagham Harbour (Cundy et al., 2002); see also Walker et al. (1991)). Some sand-dominated inlets incorporate surficial gravel deposits on the upper foreshore, but gravel is not present within the ebb-tidal shoals (e.g. North Norfolk coast at Blakeney Point and Scolt Head Island). This paper presents findings relating to the morphodynamics of a meso-tidal sand–gravel inlet at the mouth of the Deben estuary, southeast England. This system is unusual in that sediments throughout the inlet, including the ebb-tidal delta itself, comprise a mixture of gravel and sand. The inlet region of the Deben is historically dynamic, with ebb-tidal shoals in particular exhibiting broadly cyclic behaviour on a 10–30 yr timescale. Morphodynamic behaviour of the ebb-tidal delta is likely to be sensitive to any major change in estuary tidal prism, as might result from management interventions (e.g. large-scale realignment of flood defences). Ensuing changes in ebb-tidal delta and inlet morphodynamics impact downdrift coastal frontages due to changes in sediment supply and shoalderived protection from wave action. This provides a strong rationale for greater understanding of inlet and ebb-tidal delta behaviour in the context of estuary–coast interaction. This study has two main aims. First, to document and characterise the historic behaviour of the Deben inlet. Second, to evaluate this behaviour in the context of that reported for predominantly sandy systems, and to formulate a conceptual model for the morphodynamics of mixed-sediment estuarine inlets. 2. Regional setting 2.1. Geological and anthropogenic context The Deben estuary is accommodated within a northwest–southeast trending valley in Suffolk, southeast England (Fig. 1). The catchment of 163 km2 is predominantly formed in weakly consolidated Pliocene– Pleistocene shelly marine sands and clays (the Coralline and Red Crags) underlain by Tertiary London Clay (Funnell, 1996; Kendall and Clegg, 2000). During lower sea-levels of the mid-Holocene, the Deben valley is believed to have been a tributary to the Thames, and

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Fig. 1. Location of the Deben estuary, Suffolk, southeast England. Inset shows inlet region with the MLWS (mean low water springs) contour.

to have merged at that time with the Stour and Orwell to the southeast of the present port of Felixstowe (D’Olier, 1972; Brew, 1990). As sea-level rose, the valley infilled with marine silts and clays and the estuary acquired a unique mouth in the vicinity of a natural constriction in valley planform between the higher terrain of Felixstowe to the south and Bawdsey to the north. The offshore seabed comprises a mixture of mud, fine sand and broken shell. The bathymetry is partly influenced by outcrops of London Clay and sub-marine river channels, now buried and filled with fine material (HR Wallingford, 2002). Long and narrow tidal-stream aligned sandbanks occur offshore. The banks are formed of sand, supplemented with shell material likely derived from Red Crag deposits. Bottom sediments within the middle and upper reaches of the estuary are predominantly muddy, although localised accumulations of shelly sands and gravel (notably immediately northwest of Ramsholt) derive from outcrops of the

Coralline and Red Crag. Coarser sediments occur in the inlet region. The estuary has been significantly modified through the reclamation, completed during the early 19th century, of more than 2000 ha of high intertidal mudflat and saltmarsh, equivalent to nearly 25% of the tidal area (Beardall et al., 1991). The most pressing engineering and management relate to the viability of more than 25 km of defences which protect 16 discrete compartments of former estuary floodplain (N 1400 ha in area) from tidal inundation. The areas protected represent a potential additional tidal prism of approximately 12  106 m3. Many of the defences are in a poor state of repair and realignment to restore tidal action within selected compartments is one option considered as part of a recent strategic flood defence management review (Posford Duvivier, 1999). However, a large increase in tidal prism may have implications both for the behaviour of the ebb-tidal delta and the stability of the downdrift Felixstowe shoreline.

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2.2. Physical process regime This stretch of the Suffolk coast is meso-tidal and mean spring tidal range varies from 3.2 m at Felixstowe Ferry to 3.6 m at Woodbridge (Hydographic Office, 2000). The Deben estuary has a tidal length of about 18 km and a mean spring tidal prism of approximately 17  106 m3. Peak spring tidal discharges through the narrow inlet exceed 2000 m3 s 1. Since the mean flow of the River Deben 2 km upstream of the tidal limit is only 0.74 m3 s 1 (NRFA (2004) Naunton Hall: 1961– 1990; Kemp et al., 1999), the estuary is well-mixed. The coast is subject to the storm wave climate of the southern North Sea. Offshore wave heights average 0.96 m, with modal directions from the northeast (50%) and southwest (32%) (CEFAS (2004) West Gabbard Buoy: Jan–Dec 2003). Northeasterly waves are implicated in the large-scale littoral drift pattern (HR Wallingford, 2002), although strong tidal streams control the alignment of a series of offshore sandbanks. Little wave propagation occurs through the estuary inlet, and the inner estuary experiences only locally generated fetch-limited wind waves. Coastal sediment circulation is dominated by the transport of coarser material in a southerly direction as littoral drift. Finer sediments are generally transported offshore (HR Wallingford, 2002). The spit complex of Orford Ness to the north represents a significant store of coarse sediment sourced from the cliffs of Dunwich (HR Wallingford, 2002). Both Orford Ness and the Deben inlet impart a degree of intermittency to the long-term littoral drift pattern, whilst facilitating bypassing of the Alde/Ore and Deben estuary mouths. Estimates of the littoral sediment flux in the vicinity of the Deben vary, but the wide-ranging review by HR Wallingford (2002) indicates a likely transport of sand and gravel between 20  103 and 40  103 m3 yr 1 between the Deben and Languard Point (Felixstowe). 2.3. Morphology The tidally dominated middle and upper reaches of the estuary are characterised by a single meandering channel, flanked by muddy intertidal flat and saltmarsh. Upstream of Martlesham Creek, the estuary is almost entirely intertidal. Seaward of Waldringfield, the subtidal channel is deeper, exceeding 10 m below Mean High Water Spring tides (MHWS) in several places. Immediately landward of the estuary mouth, the channel divides around a large shoal, Horse Sand, which is partly intertidal. The main inlet channel between Bawdsey and Felixstowe Ferry is, at only 180 m in width, the

narrowest section of the estuary seaward of Woodbridge. Immediately offshore, the course of the subtidal channel is defined by the position and extent of a historically mobile system of intertidal shoals known locally as The Knolls (Fig. 1 inset). The inlet region of the Deben can thus be conceptualised in the manner of Hayes (1975) in terms of a landward flood–tidal delta (Horse Sand), and a seaward ebb-tidal delta seaward (The Knolls). Bathymetry within and between the channels around the flood–tidal delta implies flood dominance to the northeast and ebb dominance to the southwest of Horse Sand. Within the ebb-tidal delta, channel depth decreases over a shallow bar (often less than 1m deep at Mean Low Water Spring tides (MLWS)). 3. Methods 3.1. Approach There are three main components to the analysis. First, an analysis of all documented historic configurations of inlet planform and, where possible, bathymetry over the last 200 yr. Second, the evaluation of historic trends and variability (i.e. morphodynamic behaviour) in the context of possible mechanisms and forcing factors. Third, the formulation of a preliminary conceptual model for the behaviour of mixed-sediment inlets of this type, based upon both the envelope of historic behaviour and visual observation of morphology and processes. 3.2. Historic inlet configuration In comparison with many small estuaries in the UK, the Deben is well-documented in respect of bathymetric and other surveys. These include Ordnance Survey (OS) maps and plans; Hydrographic charts published by the British Admiralty (Hydrographic Office); and Trinity House/Harwich Haven Authority surveys (Table 1). OS maps are particularly useful for identifying changes in overall planform configuration of the inlet and ebb-tidal delta region. Hydrographic charts cover a similar range of dates and provide vital bathymetric information. Over the last decade or so, detailed surveys of the inlet and the Deben–Felixstowe frontage have been conducted by Trinity House and Harwich Haven Authority. These near-annual surveys are necessitated by the dynamic nature of the inlet shoals and the requirement to ensure safe navigation by recreational and small fishing craft. All charts and maps were scanned and geo-rectified to Ordnance Survey (OS) Grid. Martello Towers, built between 1805 and 1812 as part of defences during the

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Table 1 Coverage of surveys of the Deben estuary Map/chart date (survey date(s)) Ordnance survey

1805 1853 1881 1889 1903 1926 1946 1955 1999 Published admiralty charts 1819 1847, 1868 (1847) 1858 1879 (coast 1872–1878; inlet 1873) 1892, 1893, 1894 (coast 1892; inlet 1913, 1914 (1913) 1934 1950 (1950) 1961 (1960) 1972 (1969) 1981 (1965–1978) 1984 (1972–1981) 1987 (1985–1986) 1990 (1985–1989) 1999 (1998) 2002 (2000) Trinity soundings 1991 (17/18th June 1991) 1992 (7/8th May 1992) 1993 (8/9th June 1993) 1994 (12/13th June 1994) 1995 (3rd/4th April 1995; 31st August 1995) 1996 (20th/21st April 1996; 31st 1st June 1996; 16/17th November 1997 (10th, 12/13th April 1997) 1998 (12/13/14th March 1998) 1999 (31st March, 1st/2nd April 2000 (20th, 22nd, 23rd April 2000) 2001 (12/13th April 2001) 2002 (31st May 2002; 10th and 16th August 2002) 2003 (24th January 2003)

Scale Old series, 1st edition 1W–1 mile (1 : 63 360) Old series, hill-shaded 1W–1 mile (1 : 63 360) 25W county series: 1st edition (1 : 2500) 6W county series: 1st edition (1 : 10 000) 25W county series: 2nd edition (1 : 2500) 25W county series: revised edition (1 : 2500) Provisional edition (1 : 25 000) Provisional edition (1 : 25 000) Landline.Plus (edina.ac.uk/digimap) Medium Medium Low High 1893) High High Low Medium Medium Medium Medium Medium Medium Medium Medium Medium High High High High High

Datum OD Newlyn

MLWS MLWS MLWS MLWS MLWS 1V below MLWS 1V below MLWS LAT LAT LAT LAT LAT LAT LAT LAT LAT Chart datum Felixstowe pier (6LAT)

May, High 1996) High High 1999) High High High High High

[OD = Ordnance Datum; MLWS = Mean Low Water Springs; LAT = Lowest Astronomical Tide].

Napoleonic Wars, provided essential control for the geo-referencing of earlier charts that lacked projectional information. Additional control detail was derived from the latest OS digital map product (Landline.Plus). All levels were reduced to Ordnance Datum (OD) Newlyn. Data were integrated in a GIS and specific features of interest extracted, including low and high water positions (for delineation of intertidal sediment bodies), and bathymetry (for estimation of landform feature volumes). Despite the plethora of survey information, this analysis is subject to the usual limitations associated

with the synthesis of information from source materials of variable quality and resolution and irregular time sampling interval (e.g. Fenster et al., 1993). Horizontal errors associated with the rectification never exceeded 15 m (RMS), and averaged about 9 m (RMS). The accuracy of the original bathymetric surveys is more difficult to quantify. A critical assessment of historical bathymetric changes in the Ribble estuary (Lancashire, UK) by van der Wal and Pye (2003) suggests that only changes greater than F0.5 m can be considered significant. A similar criterion is adopted here.

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3.3. Analysis of morphodynamic behaviour A positive residual (dno-deltaT) approach was used to determine ebb-tidal delta volume (e.g. Stauble, 1998). Bathymetric data were interpolated onto a regular 10  10 m grid covering the inlet region (approximately 3.8  106 m2), using a cubic triangulation method (Barber et al., 1996). A delta area was defined explicitly for each dataset, within which no-delta contours were extrapolated and used to create corresponding no-delta baseline grids. Long-term shoreline recession and nearshore steepening precluded the use of one common no-delta surface. Integration of the positive residuals between each pair of raw and no-delta grids yielded ebb-tidal delta shoal sediment volumes. Previous studies have not included any comparison of up- and downdrift shoal volumes, although various authors have addressed the calculation of specific inlet and ebb-tidal delta features (e.g. Kraus, 2002; VilaConcejo et al., 2003). To this end, ebb channel and ebb-jet position within the inlet were digitised from the bathymetric grids through identification of the thalweg position from inlet throat through the ebbtidal delta and extrapolated to the grid boundary on the basis of maximum slope. Using the digitised thalwegs to define the divide between up- and downdrift components, positive residual volumes were recalculated for the individual up- and downdrift elements to quantify shifts in relative volumes between the north and south margins. Additional aspects of inlet morphology were extracted from the gridded bathymetries, including the envelope of channel variability; ebb channel length and ebb-jet angle; throat mean sea-level (MSL) cross-sections; and inlet channel long-sections. Fixed shore– normal nearshore transects, updrift and downdrift of the inlet, were also extracted (Fig. 1). 3.4. Mechanisms and forcing factors Supporting data were obtained from published reports, specifically the Southern North Sea Study (HR Wallingford, 2002) and FutureCoast (DEFRA, 2001), which provide regional detail, including offshore geology and large-scale sediment transport. A consultancy report to Hutchison Ports (UK) by Posford Haskoning (2003) provided useful information relating to the downdrift coastal behaviour. These studies provide indicative sediment transport pathways and fluxes, derived mainly from large-scale coastal modelling. Sea-level data were obtained from the UK Permanent Service for Mean Sea Level (PSMSL). The record

for Felixstowe is of poor quality and the closest tide gauges yielding reliable trends are Southend (70 km south) and Lowestoft (60 km north). Short-term wave data for the southern North Sea were obtained from CEFAS (2004) and regional climate data (including wind speed and direction) were obtained for Meteorological Office stations at Woodbridge, Wattisham and Walton-on-the-Naze. Contextual information regarding contemporary morphology and processes was gathered from analysis of recent aerial photography and field visits. Qualitative assessment of gross sedimentology and discrete sampling at specific subtidal (grab sampling (N = 11)) and intertidal (surface sampling (N = 10)) locations throughout the inlet region provide a quantitative basis for sedimentological comparison with other systems. Samples were sized at 1/ intervals (dry sieving) and the median grain size (D 50) for each was obtained graphically. 4. Results 4.1. Historical configuration and behaviour Post-1819 morphological evolution of the Deben inlet and ebb-tidal delta shoals, as derived from the analysis of bathymetric data, is presented in Fig. 2. Over this 184 yr period, the throat region of the inlet has maintained a relatively stable position. Gross morphological variability is confined to the ebb-tidal delta and the channel within it. In 1819, the ebb-tidal delta comprised a single long bar extending from the Bawdsey foreland, which constrained the south-trending ebb channel against the Felixstowe Ferry shoreline. Close to the downdrift tip of the delta, the channel skewed southeast, bounded to the south by a smaller shoal attached to the Felixstowe shoreline. By 1847, the deep section of the main ebb channel had shortened: the ebb-jet region was in a relatively similar position, but was larger and comprised a bifurcated channel and associated midchannel ebb shoal. Breakdown of the ebb-tidal delta form continued throughout the 1800s and early 1900s. The deep portion of the low tide channel progressively shortened, whilst the ebb-jet region moved northward, accompanied by fragmentation of the ebb-tidal shoals. In 1913, there was little evidence of the extended bar form of the 1819 ebb-tidal delta: the ebb-jet region was located 400 m further north (halfway between the two Martello Towers) and the more fragmented upand downdrift ebb shoals were dissected by smaller marginal channels.

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Fig. 2. Historical bathymetries for the Deben inlet and ebb-tidal delta. Location of Martello Towers included for ease of cross-comparison. 29

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The 1950 configuration provides the next evidence of an extended updrift ebb shoal. Though not entirely comparable with the 1819 morphology, this system comprised similarly located features: the ebb-jet was in a southerly position, the updrift ebb shoal was relatively extensive and a downdrift shoal extended eastward from the Felixstowe shoreline. However, the southern part of the main channel had significantly shallowed, leaving only a short reach of deep water through the throat area. In addition, the downdrift shoal had prograded eastward creating a notable foreland fronting the southern Martello Tower. Evidence from the 1946 and 1955 OS maps, constrained by an aerial photograph from 1948 (Arnott, 1968), confirm that extensive sea defence works (groynes and vertical sheet piling) were constructed along the Bawdsey shoreline between 1946 and 1948 and on the Felixstowe shoreline before 1946. This may account for the presence of a broader foreland fronting the updrift margin than was evident in previous surveys. An extended updrift shoal is again present in 1969. In the intervening period, there is clear evidence of a more fragmented ebb-tidal delta structure. In 1961, the main channel had not changed much from 1950, but both upand downdrift shoals were relatively small and the ebbjet divided before entering open water. By 1969, the updrift shoal had prograded significantly, causing the low tide channel to hug the Felixstowe Ferry shoreline, with a significant change in ebb-jet direction. The downdrift foreland had decreased in size considerably, allowing the channel to move further west. Survey frequency increases in the latter part of the 20th century, providing greater insight into the changing inlet and ebb-tidal delta configuration. Whilst comparable dstatesT can be identified, such as the dominant presence of an extended updrift shoal seen in 1819 and 1969 and the fragmented delta morphology of 1913 and 1961, the periods between these states, and the probable nature of the shifts in configuration are ambiguous. Progressing through the 23 yr from 1978 to 2001, the ebb-tidal delta appears to display a gradual change from the fragmented, breakdown state of 1978 to the elongated updrift shoal configuration of 2001. Three characteristic system states are evident in this period: 1: shortened, breached and degraded (e.g. 1981) 2: dominant updrift longshore bar (e.g. 1992) 3: dover-extendedT, prograded (e.g. 1999). Transition from State 1 to State 3 is marked by a phase of extension (State 2), where sediment is added to

the updrift ebb shoals, which gradually prograde southward. The two years following 2001 show the delta experiencing breakdown, whereby the ebb-jet shifts northward and changes orientation to the east. Between 2002 and 2003, a new terminal lobe forms in the northerly ebb-jet position. Multiple surveys between April 1995 and April 1997 provide evidence of the shorter timescale changes in configuration (Fig. 3). Changes are discrete and confined almost entirely to the distal region of the ebb-tidal delta, in the vicinity of the ebb-jet. From April to August, 1995, small-scale accretion on the intertidal updrift shoals forced a slight re-orientation of the ebbjet further southward. Between April 1995 and April 1997 the entire ebb-jet region shifted westward and the direction of the ebb-jet changed from southeast to south trending. More widespread shallowing of the bathymetry within the nearshore zone of the ebb-tidal delta occurred between August 1995 and April 1996. The intertidal portion of the shoals also became more coherent over the course of the 1995–1996 winter. This implies an inlet-wide influx of sediment. If just the 1992 and 1996 are considered, the progradation phase appears to be continuous. Intervening surveys suggest, however, that the system first undergoes a small-scale intertidal breakdown with a notable loss of intertidal volume, followed by reconstruction and resumed growth. The fact that the subtidal region continued to prograde and migrate, whilst the intertidal region did not, suggests that the morphodynamics of these two elements of the system are not tightly coupled. 4.2. Trend and variability 4.2.1. Ebb channel The inlet thalweg provides a clear delineation between the up- and downdrift components of the ebbtidal delta. The envelope of channel position is large (Fig. 4a). The channel has experienced changes in length (defined by distance to the 4 m OD depth contour) of over 900 m, whilst ebb-jet orientation has shifted over 1208. When a standardized 15–20 yr timescale is used (based on frequency of pre-1950 surveys), variability is much reduced: channel length varies by less than 500 m and ebb-jet orientation by less than 908 (Fig. 4b). This appears to be the characteristic time scale for the natural ebb-tidal delta dynamics. The wider envelope of variability is largely attributed to the most recent morphodynamic cycle (Fig. 4c). The channel exhibits the progressive downdrift migration previously identified, followed by a sudden shift (relocation), to the north. Whilst there is no reason to

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Fig. 3. Bathymetries for the Deben inlet and ebb-tidal delta over the 1994–1997 period. Location of Martello Towers included for ease of crosscomparison.

suppose that this degree of change could not have occurred during any of the preceding more sparsely surveyed interludes, it is important to note that the southerly extension of the 1990s is far beyond that recorded previously. Indeed the daccommodation spaceT for this southerly extension has only been acquired in the latter

half of the 20th century, following progressive recession of the downdrift shoreline. Channel length and ebb-jet angle are highly correlated with shifts in channel position (Fig. 5a). The ebb channel increases in length as it becomes forced further downdrift, whilst the ebb-jet angle rotates from an east-

Fig. 4. Channel variability within the ebb delta region of the Deben inlet.

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Fig. 5. Historical change in (a) channel length and ebb-jet angle and (b) throat cross-sectional area and bar depth, within the Deben ebb-tidal delta.

erly direction (obtuse to the shoreline) to a more southwesterly direction (acute to the shoreline). Although the channel throat has occupied a relatively stable position, its cross-sectional area has varied, with a notable increase in the latter half of the 20th century (Fig. 5b). The channel long-profile has varied in accordance with changes in channel position, but throughout the surveyed history, the minimum depth over the bar has remained relatively stable at around 2.4 m OD (approximately 0.6 m below MLWS). 4.2.2. Nearshore and ebb delta shoals Long-term shoreline recession and steepening of the nearshore zone, both north and south of the inlet, are evident in nearshore profiles up- and downdrift of the inlet (Fig. 6). Substantial erosion of the foreland fronting the southern Martello Tower has occurred, with a maximum shoreline retreat of 280 m between 1819 and 2003. The majority of this retreat occurred during the early to mid-1800s, and despite small episodes of growth, the overall trend has been recessional. Updrift of the inlet (Fig. 6a), the nearshore slope has changed

from 1.18 in 1819, to 1.88 in 1913 and 3.78 in 2003, with a comparable increase of 1.78 on the downdrift shoreline (Fig. 6b). Over the same period, the entire shoreface in the vicinity of the inlet has deepened by up to 2 m, consistent with regional steepening (Taylor et al., 2004). Downdrift movement of the ebb-jet region, typical of the late 1990s, is manifest in the nearshore as a gradual onshore shoal migration (Fig. 6c): between 1995 and 2001, the crest moved onshore at an average rate of 50 m yr 1 and increased in elevation. As the shoal migrated shoreward, the channel, already constrained between shoal and main shoreline, became more incised: continued movement of material onshore between 2001 and 2003, caused shallowing and narrowing of the channel, and hence updrift relocation of the ebb-jet region. 4.2.3. Ebb-tidal delta volume The overall sediment budget of the Deben ebb-tidal delta system has varied by F 70% around a 180 yr mean volume of 780  103 m3 (Fig. 7a). Variability is largest within the most recent 30 yr, which show an increase in

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Fig. 6. Historical change in cross-shore profiles of the (a) updrift and (b) downdrift nearshore zone within the Deben inlet region, and (c) recent changes in the downdrift nearshore zone. See Fig. 1 for transect locations.

delta volume. The coarse temporal resolution of survey data throughout the 19th and early 20th century potentially masks earlier variability. The ratio between downand updrift shoal volumes provides a further insight into historical behaviour (Fig. 7b). The shortened, breached and degraded form (State 1) corresponds to a high down–updrift ratio (1893, 1961, 1981, 2003) and the extended, prograded form (State 3) to a small ratio (1819, 1847, 1950, 1998–2002). Progression from State 1 and State 3 is gradual (about 20 yr in the most recent cycle), in comparison to the breakdown and shift back from State 3 to State 1 (2 yr in the most recent cycle).

The overall historical increase in ebb delta volume is due as much to an increase in shoal area as to vertical accretion. Downdrift migration of the main ebb channel, driven by longshore sediment transport to the updrift shoal, causes an increasingly larger region of the shoreline to be contained within the ebb-tidal delta. The increased volume throughout the recent decade, in comparison to previous decades, is attributed to the channel shifting considerably further down the coastline than had been possible previously. Calculation of up- and downdrift shoal volumes also allows quantification of sediment transport rates. The 1981–2002 period exhibits clear opposing trends bet-

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Fig. 7. Historical changes in a) positive residual ebb-tidal delta volume and b) the ratio between downdrift (V downdrift) and updrift (Vupdrift) shoal volumes.

ween up- and downdrift (Fig. 8). Growth of the updrift component is facilitated through an accretion rate of 38.6  103 m3 yr 1, whilst the downdrift component shows a removal rate of 30.1  103 m3 yr 1. This implies a longshore sediment transport rate of approximately 30– 40  103 m3 yr 1. One further point relating to these

changes is the variable behaviour during the mid-1990s. The period between August 1995 and April 1996 shows a notable increase in both up- and downdrift shoal volumes. This influx of sediment is reworked over the following year, and by April 1998, the system had returned to transporting rates more comparable to pre-1996.

Fig. 8. Cumulative change in ebb delta volume (1981–2002).

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5. Analysis and discussion 5.1. Inlet and ebb delta character Quantification of inlet morphological and hydrodynamic parameters is clearly useful in examining timeseries of change, but also allows comparison of the Deben inlet and ebb-tidal delta with other systems. A meso-tidal regime and storm wave climate indicate that the south Suffolk coastline is tide-dominated (Davis and Hayes, 1984) (Fig. 9), conditions generally considered ideal for tidal delta formation (Hayes, 1975). Comprehensive classification of inlets is hindered by their morphological diversity (Seabergh, 2002), but the systematic hydrodynamic control on specific morphometric parameters has allowed for the development of predictive models. Empirically derived models of inlet character include the inlet prism–area relationship of O’Brien (1931, 1969) and more recent schemes linking prism and littoral drift (Bruun and Gerritsen, 1960), prism and ebb delta volume (e.g. Walton and Adams, 1976; Hicks and Hume, 1996), prism and bypassing interval (Gaudiano and Kana, 2001), and wave–tide climate and bar minimum depth (Buonaiuto and Kraus, 2003). 5.1.1. Inlet morphometry Inlet morphology is dependent on the relative strength of tidal and wave energies, and to some extent sediment supply (FitzGerald, 1988). Some aspects of

35

these interactions are amenable to empirical generalisation. O’Brien (1931) found that inlet throat area could be related to tidal prism through a power law (Fig. 10a), whereby an increase in tidal prism yields an increase in throat area. This model has since been modified to account for variability attributed to wave climate and inlet structures (Jarrett, 1976), and the distinctive characteristics of particularly small systems (Byrne et al., 1980) (Fig. 10b). The Deben plots well within the range of existing inlet data and area–prism relationships (Fig. 10). However, data from the recent high resolution (Trinity) surveys indicate greater variability in the inlet throat cross-sectional area than would be expected on the basis of the recorded variations in tidal prism (mean of spring tidal prisms within the month prior to each survey, derived from Felixstowe tidal data). This behaviour is most evident at smaller throat areas which are generally not associated with a comparable decrease in tidal prism. Walton and Adams (1976) showed that a power law could also be used to describe a positive dependence of ebb-tidal delta volume on tidal prism (Fig. 11a). They also noted the importance of wave energy in reducing the efficacy of tidal currents through the delta, such that high energy wave climates are associated with reduced offshore extent and size of shoals (Dean and Walton, 1975). For a given spring tidal prism, the Deben inlet has a smaller ebb-tidal delta volume relative to the range of previously published inlet data (Fig. 11a).

Fig. 9. Wave- vs. tide-regime: Deben value refers to MTR and Hs50, with derrorT bars showing MNTR–MSTR range and Hs25 to Hs75. For explanation, see text. CIRP refers to the Coastal Inlets Research Program Database of Federal Inlets and Entrances.

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Fig. 10. Relationship between inlet throat cross-sectional area and tidal prism in the context of a) other inlets and the O’Brien (1931) power law and b) the Jarrett (1976) and Byrne et al. (1980) power laws. [Deben data relate to the 1991–2003 high resolution surveys. Other data obtained from Bruun (1966), Buonaiuto and Kraus (2003), Cayocca (2001), CIRP (2004), Gao and Collins (1994), Gaudiano and Kana (2001), Hicks and Hume (1996), Hume and Herdendorf (1992), Johnson (1973), Kana et al. (1999), Marino and Mehta (1987), Smith and FitzGerald (1994), Vandever and Miller (2003)].

Indeed, on the basis of the Walton and Adams (1976) wave energy criterion, the Deben would be classed as a high wave energy system even though the coastline is tide-dominated (Fig. 9).

Hicks and Hume (1996) found that New Zealand inlets could be better characterised when the angle between ebb-jet and shoreline was also considered. Wave processes were found to be more effective at

Fig. 11. Relationship between ebb-tidal delta volume and tidal prism (X), in the context of a) high (*) and low (**) wave energy (Walton and Adams, 1976) and b) ebb-jet angle (Hicks and Hume, 1996). [Deben data relate to the 1991–2003 high resolution surveys. Other data obtained from Buonaiuto and Kraus (2003), CIRP (2004), Cleary (2002), Cleary and FitzGerald (2003), Davis and Fox (1981), Elias et al. (2003), FitzGerald (1984), Gaudiano and Kana (2001), Hicks and Hume (1996), Hume and Herdendorf (1992), Kana et al. (1999), Marino and Mehta (1987), Oertel (1988), Smith and FitzGerald (1994)].

H. Burningham, J. French / Marine Geology 225 (2006) 23–44 Table 2a Inlet stability classes based on the ratio of tidal prism (X) to average annual littoral drift (M) (after Bruun and Gerritsen, 1959, 1960; Bruun, 1966) X /M

Sediment bypassing

Stability condition

Inlet characteristics

N300

Poor dbarT bypassers

Good

Stable, deep channels, little shoaling Some shoaling Much shoaling, unstable entrance, prone to closure

100–300 b100

Good dbarT bypassers

Fair Bad

counteracting offshore tidal current sediment transport where the angle between ebb-jet and shoreline was acute, resulting in smaller ebb-tidal deltas (Fig. 11b). The combination of delta volume and tidal prism for the Deben is indicative of acute angles (10–408). Historically, the Deben ebb-jet has undergone shifts through 1308 with respect to the downdrift shoreline, but is not specifically associated with low angles. Although the ebb-jet is more often oriented closer to shore-normal, the axis of the main ebb channel does tend to maintain a course near parallel to the downdrift shoreline. For most parts of the delta system, therefore, wave processes are potentially effective at transporting material onshore. This is an important consideration with respect to the northeast dominated wave direction on this shoreline. Orientation of the main shoreline, in the vicinity of the inlet, changes from 558N (NE–SW) to 08N (N–S), and back to 408N (NE–SW) south of the inlet (Fig. 1). Northeasterly waves, even after refraction, maintain a significant longshore component. At the inlet, where shoals follow a similar trend to the main ebb channel and shoreline (08N), waves acquire a significant onshore rather than longshore component, thereby minimising the offshore extent of ebb delta shoals up- and downdrift of the ebb-jet. 5.1.2. Inlet stability and sediment bypassing Parameters used to model the equilibrium morphology of inlets and ebb-tidal deltas can also be used to characterise their dynamic behaviour. Bruun and Gerritsen (1960) classify inlet stability on the basis of the ratio of tidal prism (X) and littoral drift (M) (Table 2a). Using a contemporary average spring tidal prism and the longshore transport rate obtained from the volumetric analyses presented here, X / M6500. This classifies the Deben as stable and a dpoor bypasserT (Fig. 12). Tidal flushing is clearly efficient in comparison to the longshore wave power such that, under current conditions, the inlet is unlikely to experience closure. Sediment bypassing is similarly categorised in terms of the ratio of M and maximum spring inlet discharge

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( Q): inlets with low ratios (10 b M / Q b 20) bypass sediment through channel transport or channel/shoal migration whereas those with high ratios (200 b M / Q b 300) bypass sediment along the periphery of the ebb-tidal delta, through wave action (Bruun and Gerritsen, 1959). Recent values of M / Q for the Deben lie in the range 15.1 to 19.3, suggesting a dominance of channel/shoal migration. The above ratios are also consistent with the historical behaviour of the Deben. Inlet throat position has remained stable, with no suggestion of longshore migration prior to the construction of the more recent coastal protection works. Channel migration associated with accretion/erosion cycles in the ebb delta shoals clearly facilitates transport of sediment from the updrift Bawdsey shoreline to the downdrift Felixstowe margin. The inlet and ebb delta system ostensibly behaves as would be expected in relation to the tidal prism and actual sediment transport rates. However, considering potential longshore sand and/or gravel transport rates, which range from 63  103 to 400  103 m3 yr 1 based on modelled wave transport (HR Wallingford, 2002), the inlet system would be less stable if it were sand-dominated rather than gravel-dominated (Table 2b).

Fig. 12. Inlet stability analysis based on the Bruun and Gerritsen (1960) classification of relative influence of littoral drift rate and tidal prism. [Deben data relate to the 1991–2003 high resolution surveys. Other data obtained from Bruun (1966), Buonaiuto and Kraus (2003), Cayocca (2001), CIRP (2004), Cleary (2002), Cleary and FitzGerald (2003), Davis and Fox (1981), Gao and Collins (1994), Gaudiano and Kana (2001), Hicks and Hume (1996), Hume and Herdendorf (1992), Johnson (1973), Kana et al. (1999), Marino and Mehta (1987), Smith and FitzGerald (1994); Vandever and Miller (2003), Wilhoit et al. (2003)].

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Table 2b Comparison of drift rates and resulting inlet stability for the Deben Littoral Drift Rate ( 103 m3 yr 30.1–38.6 210 (sand) 400 (sand) 8.5 (gravel) 141 (sand and gravel) 62.7 (sand and gravel)

1

)

X /M

Source (* cited in HR Wallingford (2002)) This study Bawdsey (Onyett and Simmonds, 1983*) Felixstowe (Onyett and Simmonds 1983*) Bawdsey (HR Wallingford, 1997*) Bawdsey (Posford Duvivier, 2000*) Felixstowe (Posford Duvivier, 2000*)

Inlets that exhibit sediment bypassing maintain a dynamic equilibrium whereby shoal migration over specific timescales facilitates longshore transport whilst maintaining a tidal exchange between estuary/basin and coast. Examination of South Carolina inlets has shown that the timescale associated with sediment bypassing is variable and related to inlet size (Gaudiano and Kana, 2001). Neglecting wave climate, Gaudiano and Kana (2001) found a positive relationship between bypassing cycle and tidal prism (Fig. 13a), whereby larger systems required longer cycles than smaller systems. Tidal prisms and ebb delta volumes of the inlets considered by Gaudiano and Kana (2001) span several orders of magnitude. These have bypassing intervals in the range 4.1 F 2.6 to 7.6 F 2.8 yr, which are consistent with cycle lengths reported elsewhere (FitzGerald et al., 2000). The Deben is notably different (Fig. 13). Its most recent cycle (totalling 22 yr) is more than 3 times longer than cycles at other inlets with comparable tidal prism (Fig. 13a) and more than 5 times

Map & chart analysis

t

Hydrodynamic modelling

442–567 (stable) 81 (unstable) 43 (unstable) 2007 (stable) 121 (some shoaling) 272 (near stable)

longer than inlets with comparable ebb delta volumes (Fig. 13b). Given that the Deben ebb delta volume is smaller than would be predicted for the size of inlet, the cycle interval should be shorter than expected. The fact that it is significantly longer implies that the sediment transport processes are less efficacious than at other inlets. The inlets presented by Gaudiano and Kana (2001) are sand-dominated, whilst the Deben is mixed gravel and sand, a characteristic seemingly more important to inlet and ebb delta behaviour than previously recognised. The intermittent nature of the earlier surveys precludes any direct identification of periodicity, but anecdotal evidence suggests that the cycle has lengthened since the 1960s, from a reported 9–14 yr period (Simper, 1992). It is impossible to corroborate this with the data available but interestingly, the delta has increased in volume since the 1960s, which is shown by Gaudiano and Kana (2001) to be associated with an increase in cycle interval. Thus, while the Deben ebb

Fig. 13. Inlet shoal bypassing interval in the context of the inlet size parameters a) tidal prism and b) ebb delta volume. Data from the Deben (relating to the most recent cycle) is compared with the work of Gaudiano and Kana (2001), who presented a simple linear relationship between tidal prism (X) and bypassing interval (I), shown in (a) [I = 0.046X + 4.56].

H. Burningham, J. French / Marine Geology 225 (2006) 23–44

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delta is not directly comparable to sand-dominated inlets in terms of size, similar underlying principles ostensibly govern its behavioural response to relative changes in process. 5.1.3. Sedimentology A clear reason for the distinctive morphological behaviour of the Deben ebb-tidal delta lies in the incorporation, within the ebb shoals, of a significant gravel component. Sandy gravel (D 50 = 2.5/, 5.5 mm; 25% sand, 75% gravel by weight) is found throughout the ebb-tidal delta and adjacent coastal beaches of the Deben, whilst gravel-sized material floors the channels (D 50 = 5.4/, 42.2 mm). The flood–tidal delta (Horse Sand) is largely composed of shelly medium sand (D 50 = 1.3/, 0.4 mm). Ebb delta sediments are frequently sorted, which often appear as significant patches of dpureT gravel or sand. Exploratory trenches (to depths of 0.5–1 m) showed coarser (D 50 = 2.8/, 6.7 mm) fractions dominated the upper foreshore and supratidal crests (to depths of 20–40 cm), while sand (D 50 = 0/, 1 mm) fractions often occur as thin (5–10 cm) surfaces on the mid-foreshore. At depths greater than 0.5 m below the surface, and throughout most of the lower foreshore, the dominant well-mixed and bimodal character was consistent: pure sand was found only as discrete, thin surficial patches. The ebb-tidal delta is almost entirely derived from updrift coastal sources, primarily Orford Ness (HR Wallingford, 2002). Sedimentologically, the delta is gravel-dominated, but perhaps more appropriately defined as mixed sand–gravel, owing to the strongly bimodal medium sand and gravel composition. The delta is dependent entirely on waves for supply, but on a combination of waves and tidal currents for its form and behaviour. Grain size is thought to exert an important control on ebb delta volume, although sedimentology is a much forgotten variable in most inlet and ebb-tidal delta literature. Coarser sediments are expected to settle more rapidly close to the ebb-jet region, whereas finer ¨ zsoy, 1986): material is transported further offshore (O this therefore restricts the area of coarser delta systems (Hicks and Hume, 1996). Evidence from existing work is limited due to the narrow range of sediment sizes generally reported (D 50 = 0.1–1 mm), but there is a general tendency for ebb delta volume to decrease with increasing grain size (Fig. 14). With reference to the broad range of ebb-tidal delta parameters considered here, the Deben is qualitatively different to systems presented elsewhere, and sedimentology is clearly a significant controlling factor. The

Fig. 14. Ebb delta volume of systems in relation to D 50 grain size. Inset summarises the mean grain size observed in the Deben ebb delta (including D 50 of sand- and gravel-dominated samples) in contrast to the mean and range of sizes presented elsewhere (Buonaiuto and Kraus (2003), CIRP (2004), Hicks and Hume (1996), Hume and Herdendorf (1992), Marino and Mehta (1987), Smith and FitzGerald (1994)).

coarse-grained nature of the system appears to exert a constraining role on the transporting capabilities of waves and tidal currents, resulting in a smaller ebbtidal delta that moves slower than would be predicted by previously published models. 5.2. Driving mechanisms 5.2.1. Long-term behaviour Whilst specific aspects of the morphology and character of the Deben inlet can be explained with reference to a wave–tide–sediment framework, linkages between behaviour and driving mechanisms are less clear. Longterm shoreline recession and steepening is most obviously considered in relation to changes in sea-level. The tide gauge record for Felixstowe Pier (5 km south) is too short to provide a reliable indication of sea-level trend, but analysis of records for Lowestoft (60 km to the north) and Southend (70 km to the south) indicates mean sea-level rise averaging 2.2 mm yr 1 since 1964 (French and Burningham, 2003) and 1.22 F 0.24 mm yr 1 between 1933 and 1983 (Woodworth et al., 1999), respectively. Throughout the 19th century, the shoreline was able to recede landward under this regional transgression, but the construction of sea defences and coastal protection throughout the last 100 yr has reduced this movement and contributed to an overall steepening of the nearshore (Taylor et al., 2004).

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The present study has shown that the ebb-tidal delta has significantly increased in volume over recent decades. Despite the limitations of the sparse pre-1950 surveys, the ebb delta has become a more prominent feature on an increasingly steep shoreface. There is no compelling evidence to suggest a linkage between shoreface steepening and increasing delta volume. Following Walton and Adams (1976), a more likely cause for increased volume is an increase in tidal prism. Historic change in the tidal prism of the Deben estuary has been dominated by the effects of land claim, though this was largely completed by 1847 (Arnott, 1968). The resulting reduction in tidal prism may well have contributed to the low delta volumes recorded for the mid to late 19th century. However, there has been no subsequent engineered change in estuary tidal prism that could account for the post-1960s increase in delta volume. It is possible that amplitude of the tidal range may have caused a small increase in prism. Woodworth et al. (1991) found that tidal range at Southend increased by 1.04 F 1.22 mm yr 1 (0.02 F 0.02% MSTR) between 1934 and 1966. However, such small increases in range (amounting to ~20 cm if extrapolated over the historical term considered here), would only contribute a small increase in tidal prism (~4.3 F 4.9% for springs). Changes in tidal prism due to variations in freshwater discharge are considered insignificant, where even peak river flows ( Q 10 = 1.6 m3 s 1) are small in comparison to tidal discharge (peak N 2000 m3 s 1). Furthermore, the assumption that increasing tidal prism necessarily forces an increase in delta volume has been challenged through observations on the German coast where an increase of 4.5 mm yr 1 in tidal range has been associated with a significant decrease in ebb-tidal delta volume (Hofstede, 1999). Consideration of the broader wind, wave and storm climate as a driver of long-term morphological change is equally inconclusive. There is no evidence of a direct dependency between inlet behaviour and storm surge occurrence, although the North Sea storm surge of 1953 is reported to have caused considerable flooding within the Deben and onshore sediment transport in the vicinity of the inlet (Guthrie and Cottle, 2002). The 1955 OS map implies a largely fragmented ebb-tidal delta and downdrift accretional foreland at this time, but there is no suggestion through the time-series of charts that this event had any significant impact on the long-term behaviour. Analysis of 1920–1997 wind data from Shoeburyness (Essex, 65 km to the south) by Stoodley (1998 in: van der Wal and Pye (2004)) showed an increase in the frequency of gale force winds (N 22

knots) until the mid 1970s, with an increase in the frequency of easterlies between 1973 and 1997. Bacon and Carter’s (1991) analysis of North Sea wave data showed an increase in mean wave height between 1960 and 1980. This apparent increase in wind and wave energy during the mid to late 20th century coincides with increased ebb delta volume. The stability and cyclicity of tidal inlets are known to be dependent on the rate of littoral drift, which is conditioned by both sediment supply and the transport rate. However, although increased wave energy could facilitate increased rates of transport, sediment supply has probably declined as a consequence of the increase in coastal protection throughout the 20th century. 5.2.2. Short-term behaviour The sediment bypassing cycle can be interpreted as an equilibrium mechanism that allows longshore transport of wave-driven material while maintaining tidal exchange via the inlet. In the case of the Deben, morphological change associated with this mechanism is envisaged to be progressive and driven by southerly longshore transport under a modal northeasterly wave climate. Within this context, inter-annual variability in ebb delta morphology is associated with discrete changes in wind climate and, by implication, wave energy. Analysis of wind data from Wattisham (10 km to the west) reveals a climate dominated by southwesterlies. On average, two-thirds of winds come from between 1358 and 3158, with a notable departure from this tendency in the 12 months prior to April 1996, during which over 55% of winds emanated from the northeast (N 3158 or b 1358). At the same time, ebb-tidal delta volume increased markedly, indicative of above average influx of sediment. This is entirely consistent with increased southerly longshore transport under more frequent northerly winds. 5.3. Mixed sand–gravel ebb-tidal deltas The gross morphology of the Deben inlet has experienced varying behaviour over the last 200 yr. The throat position did not exhibit any significant shifts in position prior to its partial stabilisation by defences in the late 1940s. In contrast, recession of the downdrift shoreline by up to 280 m has been punctuated by extended periods of stability and localised progradation. The overall transgression of the southern margin is also evidenced by the deepening and steepening of the shoreface in the vicinity of the inlet. The ebb delta shoals are more dynamic, displaying a number of distinct configurations that demonstrate medi-

H. Burningham, J. French / Marine Geology 225 (2006) 23–44

um-term cyclicity (bi-decadal scale) over its longerterm history. Despite the perceived historical trends of increasing ebb delta volume and inlet cross-sectional area, the system exhibits equivalent scales of variability in its short-term behaviour to that experienced over the longer-term. Cyclical behaviour is associated with the progressive extension of the updrift ebb-tidal shoals, causing downdrift migration of the ebb-jet and some recession of the downdrift shoreline. A short period of shoal breakdown terminates the growth cycle, followed by breaching of the updrift shoal and relocation of the ebb-jet to a more northerly position (Fig. 15). This process of bypassing is comparable to that of debb delta breachingT outlined by FitzGerald (1988). Wavedriven longshore transport of gravel and sand drives the downdrift extension of ebb delta shoals and diversion of the ebb channel. In the case of the Deben, this process takes approximately 20 yr. The consequent hydraulic inefficiency causes a tidal-flow driven shift in ebb-jet location, encompassing dissection of the updrift shoal, occupation of a new channel position and filling of the void left by the ebb-jet, a phase that takes approximately 2 yr to complete.

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Oertel (1977) provides a similar description of morphological change in relation to tide-dominated systems in Georgia, USA. Comparable behaviour, defined as a dmaturingT geomorphic cycle, suggests a progression from a broad dyouthfulT to a narrow dold ageT updrift shoal, which switches to a shortened shoal associated with the updrift shift in channel position. A key process identified is tidal current transport of sediment from the shoals to the ebb-jet region, which drives both the longshore elongation and the narrowing and breakdown of the updrift shoal, thereby enabling the shift in channel position. In the Deben, there is evidence of tidal scouring, but shoal character reflects a closer interaction between wave-dominated cross-shore steepened ridges that are tidally sculpted, contributing to a more irregular morphology (Fig. 1 inset). Bypassing over the shorter timescale, as evidenced by the more frequent surveys of the mid-1990s, does involve gradual extension of the updrift shoal and minor changes in the orientation and position of the ebb-jet region. However, these appear to be confined to the seaward and updrift margin of the shoals, and are therefore presumably driven by longshore supply of sediment to the updrift shoal. Late-stage breakdown of the updrift shoal is

Fig. 15. Model of ebb delta bypassing and associated shifts in ebb channel and tidal shoals for the Deben.

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possibly analogous to gravel barrier cannibalisation (see Orford et al. (2002)), whereby sediment available for longshore extension is sourced from the updrift portion of the shoal, and hence wave-driven and not tidal-driven. Again, despite its tide-dominated classification, it is clear that the behaviour of the Deben inlet is not entirely consistent with that of a tide-dominated system. The mixed sedimentology and gravel-rich nature of the Deben inlet and ebb-tidal delta is as important to the dynamics and character of the system as the tide and wave regime within which it exists. Transport of coarse-grained sediments is dominated by bedload, and usually associated with breaking waves (Carter and Orford, 1993). This does not preclude the capability of tidal currents to transport gravels, but certainly suggests that transport is dominated by waves. Tidalcurrent driven transport of sand is widely accepted as important in the morphodynamics of sand-dominated inlets (Oertel, 1977). At the Deben, the presence of gravel appears to influence the size and behaviour of the system. The ebb-tidal delta is smaller yet exhibits considerably longer cycles of sediment bypassing than expected for an equivalent sand-dominated inlet. Further examination of specific transporting processes are required to fully determine the relative roles of wave and tidal-current driven transport, but the morphological analysis presented here delivers a conceptual framework outlining the principal behaviour and driving mechanisms. 6. Conclusion Coarse-grained inlets and ebb-tidal deltas are underresearched systems, notably absent from the extensive inlet literature. The Deben inlet in southeast England is mixed sand–gravel and exists on what is technically classified as a tide-dominated coastline. Despite the significant difference in sedimentary character to inlet–delta systems reported elsewhere, the morphology of the Deben inlet is similar to its sand-dominated counterparts, comprising intertidal flood– and ebbtidal shoals. Furthermore, the sediment bypassing mechanism identified is comparable to the debb-tidal delta breachingT model defined by FitzGerald (1988). Based on empirical models defined elsewhere, the Deben ebb-tidal delta has a smaller sedimentary volume and longer bypassing interval than expected given the tidal prism of the inlet. This is largely attributed to the coarse-grained nature of the system, which reduces the transporting efficiency of both wave and tidal currents, thereby reducing the offshore extent of the shoals and

increasing the time taken to transport sediment and facilitate sediment bypassing. Characterisation of inlet morphodynamics over the timescales considered here is dependent on the availability of surveys at a high temporal and spatial resolution. The present analysis has benefited enormously from the availability of annual bathymetric surveys. Although these data do not span a full bypassing cycle, they provide considerable scope for analysis of morphology and behaviour. Continued monitoring at this resolution is essential for the advancement of morphodynamic understanding. Acknowledgements The authors wish to thank Trinity House, John White and Harwich Haven Authority for the provision of survey data, and acknowledge the helpful comments from the anonymous reviewers. References Arnott, W.G., 1968. Suffolk Estuary: The Story of the River Deben. Norman Adler & Co, Ipswich. 132 pp. Bacon, S., Carter, J.T., 1991. Wave climate changes in the North Atlantic and North Sea. Int. J. Climatol. 11, 545 – 558. Barber, C.D., Dobkin, D.P., Huhdanpaa, H., 1996. The Quickhull Algorithm for convex hulls. ACM Trans. Math. Softw. 22 (4), 469 – 483. Beardall, C.H., Dryden, R.C., Holzer, T.J., 1991. The Suffolk Estuaries. Segment Publications, Colchester. 77 pp. Boothroyd, J.C., 1985. Tidal inlets and tidal deltas. In: Davis Jr., R.A. (Ed.), Coastal Sedimentary Environments. Springer-Verlag, New York, pp. 445 – 532. Brew, D.S. 1990. Sedimentary environments and Holocene evolution of the Suffolk estuaries. PhD Thesis. University of East Anglia. Bruun, P., 1966. Tidal Inlets and Littoral Drift, vol. 2. Universitelsforlaget, Oslo. 193 pp. Bruun, P., Gerritsen, F., 1959. Natural bypassing of sand at coastal inlets. J. Waterw. Harb. Div. 85, 75 – 107. Bruun, P., Gerritsen, F., 1960. Stability of Coastal Inlets. North Holland Publishing Company, Amsterdam. 123 pp. Buonaiuto, F.S., Kraus, N.C., 2003. Limiting slopes and depths at ebb-tidal shoals. Coast. Eng. 48, 51 – 65. Byrne, R.J., Gammisch, R.A., Thomas, G.R., 1980. Tidal prism–inlet area relations for small tidal inlets. Proceedings of 17th Coastal Engineering Conference, ASCE, III, vol. 151, pp. 23 – 28. Carter, R.W.G., Orford, J.D., 1993. The morphodynamics of coarse clastic beaches and barriers: a short- and long-term perspective. J. Coast. Res. SI 15, 158 – 179. Cayocca, F., 2001. Long-term morphological modeling of a tidal inlet: the Arcachon Basin, France. Coast. Eng. 42, 115 – 142. CEFAS, 2004. Centre for Environment Fisheries and Aquaculture Science: WaveNet. http://map.cefasdirect.co.uk/wavenetmapping/ advanced.asp. CIRP, 2004. Coastal Inlets Research Program: Database of Federal Inlets and Entrances http://cirp.wes.army.mil/cirp/structdb/ structdbinfo.html.

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